This invention relates to direct bonding. More particularly, the invention relates to methods for direct bonding of a wide variety of articles and objects and devices produced by such methods.
Various methods exist for bonding glass surfaces together. These methods include, for example, wringing, fusion bonding, adhesive bonding and vacuum bonding. Bonding without the assistance of polymeric adhesives is a technology of interest for numerous industries including microelectronics and photonics. Adhesives are sensitive to thermal treatments and can fail from cycling in relatively moderate thermal environments (e.g., 0-200xc2x0 C.). On the other hand, the formation of a direct bond between two glass or metal surfaces allows for an impermeable seal that has the same inherent physical properties as the bulk material surfaces being bonded. For bonding of dissimilar materials, the resulting bond is sensitive only to CTE mismatches between materials, as compared to polymeric adhesives that typically have a CTE substantially different than at least one of the bulk substrates.
Optical wringing refers to a process of bonding glass surfaces in which adsorbed surface groups are removed from active bonds on a surface by heating the parts to temperatures typically above 600xc2x0 C. but below the softening point of the glass. Adsorbed water and organics will vaporize and the results surface sites become xe2x80x9cactive.xe2x80x9d At such a temperature or after cooling in a clean, low humidity environment, surfaces can be placed in contact at which point covalent bonds spontaneously form between xe2x80x9cactivexe2x80x9d bonds on each surface. This is similar to vacuum bonding, except the surface is activated by temperature rather than by a strong vacuum. A disadvantage of this process is the effect of high temperatures on polymers that may be associated with the glass article to be bonded, for example, fiber coatings and fiber array systems that utilize polymeric adhesives to bond the fiber array together.
Vacuum bonding involves bringing two clean surfaces into contact in a high vacuum, thus forming a bond. Provided that the surfaces are flat and clean, a high vacuum removes adsorbed water and hydrocarbons from the surface while preventing the adsorption of such species. Surfaces can be cleaved in the vacuum, processed and cleaned before being placed in the vacuum, or cleaned in the vacuum via ion milling or other plasma techniques. One disadvantage of this process is the effect of a high vacuum on polymers that may be associated with the glass article to be bonded, for example, fiber coatings and fiber array systems that utilize polymeric adhesives to bond the fiber array together. High vacuum pressure may have a negative effect on these polymers.
Within the microelectronics field, vacuum bonding has been developed for sealing of such materials as single crystal silicon, thermal oxide SiO2 grown on Si, and various metals, as described in U.S. Pat. No. 6,153,495. Coefficient of thermal expansions (CTE) mismatch between materials is not an issue because the process can be applied at room temperature.
Fusion bonding refers to the process of cleaning two surfaces (glass or metal), bringing the surfaces into contact, and heating close to the softening point of the materials being bonded (to the lower softening temperature for two dissimilar materials), thus forming a welded interface. One example of a fusion bonding process is fusion splicing of optical fibers. Advantages of fusion bonding include the fact that commercial systems exist for splicing of fibers and that the process is relatively easy to apply to bulk geometries. One disadvantage of fusion bonding is that this process typically results in deformation of the two surfaces being bonded due to the flow of softened material, the inability to use this process for complex geometries where adhesives or other low-temperature materials are used, and loss of signal transmitted through the interface when fusion bonding is used for signal transmitting objects such as optical fibers. Furthermore, for bonding of large surfaces, it is difficult to limit glass softening to the bonding interface. As a result, the entire seal can lose dimensional tolerances. In addition, the high temperature ranges required to fusion bond many glass materials are disadvantageous for complex systems that include the use of low-temperature materials such as adhesives and polymer coatings (e.g., fiber coatings).
Adhesive bonding is a common process for mounting of fibers in ferules and for bonding of photonic components such as filters, polarizers, rotators, etc. to each other and into packages. Some advantages of using such adhesive are that they are readily available, UV curable, and allow for alignment of components between application of the adhesive and curing into permanent position. Disadvantages of adhesive bonding include CTE mismatch especially for low CTE materials such as high purity fused silica, for applications where the bonded part is exposed to thermal cycling. Another issue is signal loss from transmission through the adhesive when the adhesive is used in the optical path of optical systems. Although it is possible to utilize an index matching adhesive that has a refractive index matching the optical component, it is extremely difficult to utilize an adhesive that has a CTE and refractive index that matches the optical components. In addition, there are concerns over long-term reliability of packages that incorporate adhesives. Furthermore, bonding of components with adhesives can require angle polishing (typically 8xc2x0) and associated assembly to prevent back-reflection.
Another type of bonding process involves chemical bonding. The formation of a chemical bond between two glass or metal surfaces allows for an impermeable seal that has the same inherent physical properties as the bulk material being bonded. In literature, low-temperature bonding technology has been reported for bonding soda-lime-silicate glass and for crystalline quartz (see, e.g., A. Sayah, D. Solignac, T. Cueni, xe2x80x9cDevelopment of novel low temperature bonding technologies for microchip chemical analysis applications,xe2x80x9d Sensors and Actuators, 84 (2000) pp. 103-108 and P. Rangsten, O. Vallin, K. Hermansson, Y. Backlund, xe2x80x9cQuartz-to-Quartz Direct bonding,xe2x80x9d J. Electrochemical Society, V. 146, N. 3, pp. 1104-1105, 1999). Both the Sayah and Rangsten references disclose using acid to contact the bonding surfaces. Another article, H. Nakanishi, T. Nishimoto, M. Kani, T. Saitoh, R. Nakamura, T. Yoshida, S. Shoji, xe2x80x9cCondition Optimization, Reliability Evaluation of SiO2xe2x80x94SiO2 HF Bonding and Its Application for UV Detection Micro Flow Cell,xe2x80x9d Sensors and Actuators, V. 83, pp. 136-141, 2000, discloses low-temperature bonding of fused SiO2 by first contacting the bonding surfaces with hydrofluoric acid. While these bonding processes are useful in certain applications, the bond strength provided by contacting with acidic solutions is limited and could be improved.
It would be desirable to provide a bonding process that does not have the disadvantages of fusion bonding, adhesive bonding, and wringing, and offers more reliable seal integrity than low pH chemical bonding. In addition, it would be useful to provide a bonding process that was durable, provided high bond strength and could be used on a wide variety of silicon-containing materials and surfaces.
The invention relates to methods of bonding opposing surfaces of silicon-containing articles, such as glass articles containing silica. The invention may further find use in bonding a wide variety of silicon containing materials such as single crystal silicon and crystalline quartz. According to one embodiment of the invention a method of bonding opposing surfaces of silicon-containing articles is provided. The method includes providing termination groups selected from the group consisting of xe2x89xa1Sixe2x80x94OH, xe2x95x90Sixe2x80x94(OH)2, xe2x80x94Sixe2x80x94(OH)3 and xe2x80x94Oxe2x80x94Sixe2x80x94(OH)3, and combinations thereof on the opposing surfaces of the articles and placing the opposing surfaces in contact. According to another embodiment of the invention, the temperature of the opposing surfaces can be maintained at a temperature below 300xc2x0 C. during the contacting step, resulting in high bond strength and seal integrity.
According to another embodiment of the invention, the step of providing functional groups includes contacting opposing surfaces of the articles to be bonded with a high pH solution. As used herein, the term high pH means a solution having a pH of about 8 to about 13. Suitable high pH solutions include hydroxide-based solutions such as potassium hydroxide, sodium hydroxide and ammonium hydroxide. In another embodiment of the invention, the method may further include cleaning the opposing surfaces with a detergent and contacting the opposing surfaces with an acid. In still another embodiment, the opposing surfaces may also be ground and polished prior to contacting the surfaces. According to this embodiment, it may be desirable to provide a bonding surface having a flatness less than 1 micron and a roughness of less than 2.0 nm RMS, preferably less than 1.5 nm RMS.
In a preferred embodiment of the invention, the pH of the high pH solution is greater than 8, but less than 14. In a highly preferred embodiment of the invention, the step of contacting the opposing surfaces with the high pH solution is performed after the step of contacting the opposing surfaces with the acid. Suitable acids for this step may include hydrochloric acid, nitric acid and sulfuric acid. According to still another embodiment of the invention, the opposing surfaces are rinsed with water and placed in contact without drying the opposing surfaces. In a preferred embodiment, pressure of at least one pound per square inch, more preferably, at least two pounds per square inch, is applied to the opposing surfaces during the step of contacting the opposing surfaces. In another embodiment, it may be desirable to dry the surfaces to remove absorbed water molecules and to draw a slight vacuum, for example, about 10xe2x88x923 millibar, to assist in the prevention of an air gap between the surfaces.
The method of the present invention is suitable for bonding a wide variety of silica-containing, glass and oxide-based surfaces. For example, the method could be used to bond waveguides, optical waveguide preforms, microlens arrays, optical fiber arrays, photonic components, lenses, ferrules, optical fiber waveguides, and combinations of these articles. For example, the invention may be utilized to bond two or more optical fiber waveguide fiber preforms together to provide for an enlarged fiber preform and continuous fiber drawing process. The invention may also be utilized to bond at least two glass tubes together that can be drawn into a dual fiber ferrule. The bonding method may be used to bond glass or silica-containing fibers with ferrules. The invention may also be used in the manufacture of optical fiber and lens arrays.
The invention provides a simple, low temperature, and inexpensive bonding method that provides a high bond strength. Bonding can occur at temperatures lower than 300xc2x0 C., and in some cases lower than 100xc2x0 C. The resulting seal is complete, impermeable and does not include an air gap. Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.